Among numerous applications, rubbery cushions are used as bumper buttons or strips for automobile hoods and gasoline-filler doors and as feet for small clocks and radios. For convenience of application, one surface of the cushion may have a pressure-sensitive adhesive layer and a removable covering to protect the adhesive. Rubbery cushions which have pressure-sensitive adhesive layers are often made of filled cured polyurethane resins or of thermoplastic elastomer compositions.
The term "thermoplastic elastomer" designates a material which at ordinary room temperature exhibits the mechanical properties of a vulcanized rubber but remains permanently fusible and soluble and can be processed in a conventional manner on equipment designed for thermoplastic resins. The molecular network of a thermoplastic elastomer consists of two distinct microscopically-identifiable phases, a crystalline (hard) and an amorphous (soft) phase.
Among thermoplastic elastomers, the noncuring polyurethane elastomers provide excellent rubbery cushions but are relatively expensive and can be extended only to a limited degree by cost-reducing diluents such as plasticizing oils. Thermoplastic polyester elastomers have the same advantage and disadvantages.
Another class of thermoplastic elastomers comprises simple A-B-A block copolymers where A represents a crystalline polystyrene end block and B, an amorphous polybutadiene or polyisoprene center block. The class includes more complex configurations of the same moieties. Although less expensive than polyurethane and polyester thermoplastic elastomers and more readily subject to cost-reducing addition of plasticizing oil and other diluents, these block copolymers are inferior to the polyurethane and polyester elastomers in that they tend to degrade under prolonged exposure to ozone, ultraviolet light or high temperatures due to their olefinic unsaturation.
For applications requiring both low cost and good resistance to adverse environment, a preferred class of thermoplastic elastomers comprises block copolymers having crystalline polystyrene and/or crystalline polyolefin blocks and amorphous saturated polyolefin blocks. Representative thermoplastic elastomers of this preferred class are "Kraton G" (Shell Chemical Co.), "Telcar" (B. F. Goodrich Chemical Co.), "TPR" (Uniroyal, Inc.) and "Somel" (E. I. duPont). In all of the aforementioned block copolymers, the majority of the end blocks should be crystalline.
A preferred diluent for the block copolymers is plasticizing oil, since it both reduces cost and also improves their workability and flexibility by lowering melt viscosity, second-order transition temperature and/or elastic modulus. Plasticizing oils which are most commonly used are ASTM type 103 or 104, commonly called naphthenic and paraffinic, respectively. Naphthenic oils contain less than 6% polar compounds and 35.1 to 65% saturated hydrocarbons; paraffinic oils contain less than 1% polar compounds and a minimum of 65.1% saturated hydrocarbon. Paraffinic oils are preferred from the outdoor weathering standpoint but are more migratory in thermoplastic elastomer compositions.
Rubbery cushions consisting of thermoplastic elastomer compositions including appreciable plasticizing oil have not heretofore been furnished with pressure-sensitive adhesive layers, because the adhesive tends to soften unduly through absorption of the oil within a relatively short time, thus weakening its holding power. The plasticizing oil likewise tends to weaken diverse types of adhesives which one would otherwise use to hold the rubbery cushions in place.